Microanalysis of cellular function

ABSTRACT

An inverted microwell ( 102 ) provides rapid and efficient microanalysis system ( 100 ) and method for screening of biological particles ( 128 ), particularly functional analysis of cells on a single cell basis. The use of an inverted open microwell system ( 102 ) permits identification of particles, cells, and biomolecules that may be combined to produce a desired functional effect also functional screening of secreted antibody therapeutic activity as well as the potential to recover cells and fluid, and optionally expand cells, such as antibody secreting cells, within the same microwell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/419,377, entitled “Microanalysis of Cellular Function”, filed Dec. 3,2010, which is herein incorporated by reference in its entirety for allpurposes.

BACKGROUND OF THE INVENTION

Systems for high throughput and efficient analysis of biologicalparticles, including single live cells, are needed to enable rapid andvaluable identification of particles, including cells, molecules, andthe like, that possess desired functions, for example that produce orinduce a desired biological outcome. Such systems are needed for drugdiscovery, diagnosis, screening of candidate molecules, and the like. Ina preferred system, the particle is retained in a viable form and is notdamaged significantly by the analytical process.

Current systems that evaluate single particles generally utilizedielectrophoresis (DEP) to manipulate the particles. The electric fieldimposed upon particles such as cells to levitate and maintain theparticle's position during induced reactions and analysis can harm theviability and disrupt the ability of the particle to properly function.Under prolonged use, for example, such damage can lead to lysis and/orcell death. The attachment of particles to a substrate, electrode, orwall of a chamber can also require lysis of the cell to remove it fromthe system after analysis or to capture cellular products.

It would be very useful to provide a rapid, high throughput system foreffectively and efficiently capturing, identifying, and analyzingbiological function of single particles, where the particles are notsignificantly damaged by the system.

SUMMARY OF THE INVENTION

An inverted open microwell system, device and methods of use aredisclosed herein that provide rapid and efficient screening and sortingof particles, and particularly of single particles, including singlecells. Such screening includes analysis of particular particlecharacteristics as well as functional properties of the particle thatcan indicate if a particle or reagent may be therapeutically useful. Forexample, the characteristics of the particle can include: presence andrelative amount of specific binding and/or affinity for ligands, such ascancer cell target antigens; cell to cell binding reactions andinteractions, for example lysis, toxicity, biomarker production,electroporation, and the like; induced cellular response and/orreactions, for example, induced by other cells, drugs, compounds,proteins, antibodies, molecules, enzymes, nucleic acid molecules, cellsecretions, and the like; induced production of cellular products, forexample lytic enzymes, antibodies, biomarkers, and the like. Theanalysis of these characteristics followed by particle recovery supportsmethods for functional cell sorting which lead to the isolation of cellsor their byproducts, such as secreted molecules, having a provedfunction and utility, for example, therapeutic utility.

In an embodiment, the single cell analysis can include a plurality oftests, in substantially one reaction scheme or sequentially, forexample, with the results of one test providing information for theparticular test to follow. For example, a cell known to be associatedwith a specific disease, for example, a cell obtained from an individualsuffering from the disease, can be analyzed in the system for thepresence of a particular biomarker, for example, an antigen, expressedprotein, or disease characteristic. After preliminary analysis toidentify if the single cell demonstrates the biomarker, the sameidentified cell can be analyzed again for its response to a candidatetherapeutic drug, compound, or other cellular treatment expected to beeffective to induce a desired response in cells exhibiting the screenedbiomarker.

Analysis in the inverted open microwell can be accomplished on a singleparticle, including a single cell, with rapid and efficient timing, andwithout the need to isolate the particles or products of the particlesfrom complex mixtures. Screening methods include screening of a singlecell for functional properties, with the option of recovering anoriginal cell in substantially viable form, for continued analysis,immortalization, and/or clonal expansion identified as having a usefulproperty. Cellular components such as DNA, RNA, proteins, and the like,can also be isolated from the original cell.

The methods permit the identification of desirable particle reactionsand interactions early in the development process. An earlyunderstanding of cellular function enables final development of usefulbiological materials with a better understanding of the material'spotential for success, and can reduce the number of potential candidatesearlier in the development process. Methods for multiple and rapidanalyses can be performed on the same particle, for example on a singlecell, permitting more rapid advancement in the discovery process.

Precise delivery or exclusion of single cells to microwells, and precisemanipulation of cells and other particles in the microchannel and in aspecific microwell, delivery of reagents, buffers, markers and the like,including other cells for cell-cell interactions, is facilitated by theinverted open microwell system described herein. Single cells can beevaluated for multiple characteristics, retaining vitality in the openmicrowell for optional continued testing, immortalization, expansion,and the like. Recovery of the particles, including recovery of theoriginal single cell is possible in a short time frame, using minimalreactants, and recovering cells and products in a substantially viableand useful condition. In particular, deposition of one or moreparticles, including live cells at the fluid/air interface of an openmicrowell permits precise particle to particle interactions that can beefficiently monitored and rapidly screened to identify candidateparticles, for example cells, for continued analysis and/or clonalexpansion in the microwell.

Surprisingly, fluid filling the inverted open microwell 102 having alower end 108 open to air, is retained without leakage from the openlower end when proper geometrical constraints and fluidic conditions areused. Moreover, the deposited particle(s) 128 are surprisingly retainedat the meniscus 122 during fluid washes, placement of additionalparticles in close proximity of the first, and analytical procedures.The open microwell acts as a “mini-centrifuge” permitting delivery ofreagents to the deposited particle(s) for rapid analysis, washes, andcontinued analysis of the deposited particle without loss. Cells withinthe microwell retain good viability, for example, after analysis andwhen recovered from the microwell. Recovery from the microwells can be,for example, onto substrates such as microtiter plates, for example forimmortalization and/or expansion. In an embodiment, selected singlecells can be incubated and expanded within the microwell.

The invention provides methods and structures implementing an invertedopen microwell system that comprises a microwell open at an upper end toa microchannel. In an embodiment, the microwell can be closed at a lowerend, preferably with a clear material such as glass or clear polymer topermit viewing of the contents of the microwell. In another embodimentthe microwell is open at a lower end to the atmosphere outside thedevice, for example, air or other gas.

Delivery of one or more particle to a microwell can be by sedimentation,for example, controlled by cell density, fluid speed, loading time, andoptionally dielectrophoretic forces. Dielectrophoretic forces can begenerated by electrodes connected to proper alternate voltages andpositioned in a microchannel, the microchannel positioned over themicrowell which is open and in fluid communication with themicrochannel. Also provided are methods for efficient particle focusingand interactions, controlled, for example, by electrodes embedded withinthe open microwell to create dielectrophoretic forces able to manipulateparticles, such as single cells, to a desired position within themicrowell. Such methods provide a high throughput analytical systemusing minimal reagents and permitting high-throughput recovery of viableparticles post-analysis, including cells and cell products.

The invention further provides methods and structures for guiding andsorting particles, including cells and non-cell particles, withprecision, in order to deliver a particle efficiently to a microwell ofthe inverted open microwell system. Specific embodiments includedisposition of electrodes and electrode pairs in the microchannel toenable particle movement with little or no harm to the vitality of thebiological materials, such as cells; disposition of electrodes andelectrode pairs in the microchannel and in proximity to the microwell,structures that permit controlled access of desired particles to themicrowell and that effectively close the microwell and repel unwantedparticles; and a pattern of electrodes and electrode pairs within themicrowell for detecting and optionally controlling the position of aparticle as it passes into and through the microwell, for example, fordeposition on the fluid meniscus.

The inverted open microwell system includes one (FIG. 1) or a plurality(FIG. 3) of microwells, where each microwell is in fluid communicationwith one or with a plurality of microchannels for fluid and particledelivery to the microwell(s). The microchannel is generally disposedabove the microwell, the microwell being open to the fluid microchannelat an upper end 106 of the microwell and open at a lower end 108 to theatmosphere outside the device, for example air or other gas, whoseproperties such as gas composition, humidity, temperature, and absenceof contaminants can be controlled. The microwell has a vertical axis110, for example a central vertical axis, extending between the upperend 106 and the lower end 108 of the microwell 102.

In an embodiment, the vertical wall 112 of the microwell 102 is formedat least in part by a dielectric material 114. The vertical wall mayalso be formed at least in part by one or more electrodes integratedwith a dielectric material, for example, in a laminate configurationthat is perpendicular to the vertical axis of the well, the laminateforming the microwell, for example as shown in FIG. 2.

A fluid inserted in the microchannel 104 fills the microwell 102 bycapillary action, while surface tension holds the fluid within the openmicrowell, forming a meniscus 122 at the lower open end 108 of themicrowell at the air-fluid interface. The surface of the microchanneland of the microwell can be coated with opposing hydrophilic orhydrophobic materials. For example, the microwell can have a hydrophiliccoating and the surface of the microchannel near the open end of themicrowell has a hydrophobic coating, or vice versa.

In general, movement of particles within the microchannel and placementof a particle in a particular microwell is accomplished by limitingdilution, sedimentation, electromagnetic forces, gravity, and acombination of these. In one embodiment, controlled manipulation ofparticles in the inverted open microwell system includes powering of oneor more of an array of electrodes suitably positioned in a microwell aswell as those electrodes suitably placed in the microchannels. Examplesof electrode arrangements are shown in FIGS. 2 and 9-13.

Methods disclosed herein include methods for screening single cells orsmall groups of cells, including precise aggregates of specificparticles. Analysis of single cells is generally provided to identifycells capable of producing a specific response, for example to an addedbiomaterial that may be a different cell, cell portion, protein, nucleicacid molecule, drug, antibody, enzyme, and the like. The production ofprecise aggregates permits precised alignment of cells that may togetherinduce a desired response and/or only together can be analyzed for aspecific characteristic, ability, or function. The system makespossible, for example, alignment of multiple particles in an aggregate,the cells being in direct contact or in close proximity for functionalcontact. Such alignment of cells in a precise aggregate permits rapidand efficient testing of particle-to-particle interactions.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of an inverted open microwell system,showing cells deposited at the meniscus of the open microwell with fluidmoving in the microchannel, with particle delivery from the microchannelinto the microwell by gravity.

FIG. 2 is a cross sectional diagram showing a 3 electrode configurationin an inverted open microwell system.

FIG. 3 is a schematic diagram showing an inverted open microwell systemcomprising a plurality of microwells connected to a fluidic systemproviding fluids to the microchannels and an imaging system supportingthe optical inspection of the inverted open microwell content.

FIG. 4 is a diagram showing recovery of microwell contents to amicrotiter plate.

FIG. 5 is a diagram showing recovery of microwell contents from a systemcomprising a plurality of open microwells to a recovery substrate.

FIG. 6 shows photographs of the meniscus of an inverted open microwellobserved with an inverted fluorescence microscope showing (A) K562 cellspositioned randomly at the meniscus when no electric field was appliedto focus the cells during descent and (B) K562 cells manipulated byelectromagnetic forces to a central vertical axis of the microwellduring descent and deposited as an aggregate of cells near the center ofthe meniscus.

FIG. 7 includes a graph and a series of photographs demonstrating adecrease in biomarker fluorescent signal intensity as a measure ofcalcein uptake in a living target cell over 20 minutes post exposure ofthe individual single cells to activated T lymphocytes to induce lysisof the single cell.

FIG. 8 shows a series of photographs (A,B,C) demonstrating clonalexpansion of a single K562 cell recovered from an open microwell afterdielectrophoretic positioning and transferred to V-shaped microtiterplates for expansion over 5 days: A=day 0; B=3 days; C=5 days. (D) is agraph showing an increase in relative cell number over a period of sixdays.

FIG. 9 is schematic drawing showing a top view of a representativearrangement of electrodes in the microchannel and microwell forcontrolled dielectrophoretic movement of a particle in the microchanneland at the microwell of an inverted open microwell system.

FIG. 10 is a sectional view of the schematic drawing of FIG. 9 throughline A-A′, showing an arrangement of electrodes in the microwell andmicrochannel.

FIG. 11 is a sectional view of the schematic drawing of FIG. 9 throughline B-B′, showing an arrangement of electrodes in the microchannel.

FIG. 12 shows an electrode array centered at the location of a microwellshown through the top of an inverted open microwell system anddemonstrating arrangement of electrodes in the microchannel for trappinga particle in the channel at a minimum electrical potential (Δ). Thisarrangement is most suitable when a fluid flow does not flow in thechannel.

FIG. 13 shows an electrode array centered upstream of a microwell, shownthrough the top of an inverted open microwell system, and demonstratingarrangement of electrodes in the microchannel for manipulating aparticle () toward a minimum electrical potential (Δ) established bythe specific forces applied by the electrodes.

FIG. 14 is a sectional view of one embodiment, showing an alternativearrangement of electrodes in the microchannel, containing an electrodedisposed along the top of the microchannel.

FIG. 15 shows an electrode array, according to some embodiments,centered upstream of a microwell, shown through the top of an invertedopen microwell system, and demonstrating arrangement of electrodes inthe microchannel for manipulating a particle () along F-F′.

FIG. 16 shows an electrode array, according to some embodiments,centered upstream of a microwell, shown through the top of an invertedopen microwell system, and demonstrating arrangement of electrodes inthe microchannel for manipulating a particle () along F-F′.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A. Definitions

The following terms and phrases are intended to have the definitionsshown below:

Microwell, as used herein, means a well formed with micrometerdimensions (less than 1000 micrometers), including height,cross-sectional area, for example, diameter where the microwell istubular; and volume.

Microchannel, as used herein, means a channel providing fluid to themicrowell, having a cross-sectional area of micrometer dimensions (lessthan 1000 micrometers).

Particle, as used herein, is meant to include any particle that may bedelivered, manipulated, reacted, or analyzed in the microwell of thedisclosed inverted open microwell system. The particle may be a cell orcellular portion, a microorganism, a biological molecule such as aprotein, polynucleotide, antibody, enzyme, or a substrate such as apolymeric particle that may be coated with a reactive substance, forexample, an antigen coated sphere, and the like.

Meniscus, as used herein, means the air/fluid interface formed at thelower end of the inverted open microwell by surface tension.

Electrode, as used herein, is an electrical conducting material, forexample, metal, such as gold, copper, nickel-gold, and the like.Preferred electrodes are formed of high purity gold.

Dielectric material, as used herein, is an electrical insulatingsubstrate. Preferred dielectric materials for use in the inverted openmicrowell system include polyimide, such as Kapton® and Pyrelux®,

Dielectrophoresis, as used herein, is a force exerted on a particle whensubjected to a non-uniform electric field.

“A” means at least one.

“Plurality” means two or more.

“Micro” means having at least one dimension less than 1000 micrometers.

“Comprises” or “comprising” means including at least the recitedelements or steps, and open to the inclusion of additional elements orsteps.

B. Abbreviations

The following abbreviations are used as shown below:

DEP means dielectrophoresis

ASC means antibody secreting cell

CTL means cytotoxic lymphocyte

NK means natural killer cell

LCL means lymphoblastoid cell line

ADCC means antibody-dependent cell cytotoxicity

CMC means complement mediated cytotoxicity

PCB means printed circuit board

C. Inverted Open Microwell System and Device

The inverted open microwell system includes a device (100) comprisingone (FIG. 1) or a plurality (FIG. 3) of microwells, where each microwell(102) is in fluid communication with one or with a plurality ofmicrochannels (104) for fluid and particle delivery to the microwells.The microchannel is generally disposed above the microwell, themicrowell being open to the fluid microchannel at an upper end (106) andopen at a lower end (108) to the atmosphere outside the device, forexample air or other gas. The microwell has a vertical axis (110), forexample a central axis, extending between the upper end and the lowerend of the microwell. Fluid inserted in the microchannel fills themicrowell by capillary action, while surface tension holds the fluidwithin the open microwell, forming a meniscus (122) at the air/fluidinterface.

In this application, the atmosphere outside the device at the open lowerend of the microwell is exemplified as “air”. It is to be understoodthat the atmosphere may be controlled, for example, in a chamber, tocontain gas other than air, for example, enriched with carbon dioxide,nitrogen, or other gas as desired for the maintenance of the particlesand/or analysis to be performed within the microwell. The chamber mayinclude a system to control gas composition, humidity, temperature,pressure, and/or other physical parameters and/or to maintain theenvironment in a sterile condition.

In one embodiment, a closed chamber is created by placing the devicesubstrate 114 upon a microtiter plate whose wells are aligned to theinverted open microwells. The alignment retains space between the openmicrowell and the surface of the microtiter plate such that the contentsof the open microwell and the microtiter plate, for example, fluidcontents, are separated. The microtiter wells may contain a liquidmedium, for example, a physiological medium used for cell recovery andcell culture, for example, to accept contents of the open microwell whenreleased. The microtiter wells can contain a different medium, such aswater or other fluid, for example, to increase the humidity of theclosed chamber, Increasing the humidity of the atmosphere of the closedchamber due to evaporation of liquid in the microtiter well, cansaturate vapor pressure in the chamber and prevent evaporation of fluidfrom the inverted open microwell in the chamber. In an embodiment, theatmosphere of the closed chamber is fully saturated, for example, at orabout 100% humidity, to prevent evaporation at the meniscus. Thetemperature of the substrate 114 can be maintained higher than thetemperature of the humid air contained in the chamber in order toprevent condensation on the surface of the substrate.

The inverted open microwell system can be used to achieve controlleddelivery of one or more particle, such as one or more living cell, tothe microwell, to cause interaction of a delivered particle in themicrowell with one or more additional particle delivered to themicrowell, and/or to permit high-throughput analysis of a biologicalfunction, for example, by inducing specific reactions within themicrowell, analyzing the results of the reactions, with identificationand optional recovery of selected particles and/or biological productsof selected particles.

The system can serve as a “mini-centrifuge”, with changes in the fluidcontained in the channel providing washing and reagent changes withinthe open microwell without displacement or loss of particle(s)positioned on the meniscus. In one particular embodiment, the fluidcontained in the microwell is suitable for dielectrophoreticmanipulation of particles. Once a particle is positioned in themicrowell, for example on the meniscus, the fluid dielectrophoreticmedium can be washed from the microwell and replaced with a suitableassay medium or with a medium suitable for growth and expansion of thepositioned particle.

The inverted microwell system provides efficient and effective real-timemonitoring of particle functions, including interactions of multipleparticles, functional screening, and sorting of particles, for examplelive cells. In a particular embodiment, the system provides highthroughput functional analysis and rapid identification and selection ofantibody secreting cells and high affinity antibodies, analysis of lyticactivity and selection of desired cytotoxic lymphocytes and naturalkiller cells, for example by ADCC or CMC assay. The system permits rapidanalysis of affinity and specificity of molecules secreted by singlecells, for example, monoclonal antibodies, as well as recovery ofidentified antibody secreting cells and/or secreted antibodies, and alsopermits expansion of identified cells within the microwell.

Structural attributes of the device, including relative geometries,coatings, pressures, materials, and qualities of the intended fluids,particles, and reagents to be placed in the microwell are optimized forparticular analyses to provide optimal functionality. For example, acoating applied to the interior walls of the microwell versus thatapplied to the walls of the microchannel adjacent to the microwell canbe designed to have an opposing hydrophobic/hydrophilic character torepel unwanted materials away from the well and/or to accommodate entryof a desired particle(s).

The diameter and/or length of the microwell is designed to permit theformation of a meniscus at the lower end of the microwell at theinterface of the fluid with the outside environment, for example, air,and to permit a particle or particles deposited on the meniscus to beretained with little or no leakage of fluid from the microwell, and topermit exchange of fluids and reagents in the microwell while retainingparticle(s) positioned at the meniscus. The width, height and length ofthe microchannel are designed to create a hydraulic resistance which,under specific fluid flows, produces a pressure in the microchannelwithin a range which allows the microwells to be filled while preventingthe fluid to leak from the bottom side of the microwell. These and otherfeatures of the inverted open microwell system and use are described andexemplified herein.

1. Microwell

As shown, for example in FIG. 1, the inverted open microwell device 100includes a microwell 102 having dimensions in the micrometer range. Theopen microwell can have a tubular shape, for example with a circularcross-sectional axis. Other shaped microwells can also be used, forexample, conical, rectangular, or other geometric shape. In oneembodiment the microwell is formed with an expanded width at the upperend extending from a narrowed lower end.

FIG. 1 shows a cross-sectional view of a device 100 including an openmicrowell 102. The device 100 includes a substrate 114 defining avertical wall 112 of the microwell 102 that extends transversely betweenan upper open end 106 to a lower open end 108 of the open microwell 102.

The substrate 114 can be a moldable plastic, for example, poly(methylmethacrylate) (PMMA), polycarbonate, polyethylene naphthalate (PEN),polyethylene terephthalate (PET), cyclo-olefin polymer, and the like.

The substrate 114 may have a thickness, for example, of from about 10 μmto about 500 μm, inclusive, and may be for example, 12.5, 25, or 50 μmin thickness. In some embodiments, particularly these embodimentscomprising electrodes, the substrate 114 is formed, at least in part, ofa dielectric material, for example, polyimide, Kapton®, Pyrelux®, andthe like materials.

2. Electrodes

As shown in FIG. 2, the inverted microwell system can include one or aplurality of electrodes coupled to a power source for applying voltagesto the electrodes. In various embodiments, electrodes can be arranged inthe microwell and in the microchannel to control electrical forces on aparticle, for example, to push or pull a particle in a microchanneltoward and/or into or out of a microwell, or to facilitate a particle'stransport from entering the open microwell to the lower end of themicrowell, or any portion of these.

While three electrodes 116, 118, 120 are shown in the microwell of inFIG. 2, the device 100 may alternatively include more or fewerelectrodes. In an embodiment, the microwell is surrounded by at leastone annular electrode 116. In another embodiment, a pair of facingelectrodes 116, 116A rings the microwell. In an embodiment, a pair offacing electrodes can be positioned near the upper end 106 of the openmicrowell, providing controlled entry of a particle 128 to the microwelland/or controlled positioning of a particle along a vertical axis 110,for example a central vertical axis.

Electrodes can also be arranged in the microchannel to impact transportof particles. For example, FIGS. 10-11 show possible arrangements ofelectrodes within the microchannel 155, 156, 153, 154 and within themicrowell 116, 118. FIGS. 12-13 demonstrate an exemplary array ofelectrodes 153-164 disposed in pairs along the microchannel facilitatingtransport of a particle toward the microwell 102 and away from themicrowell 102.

The electrodes are formed of a conductive material that may be, forexample, in the form of conductive sheets or plates. In someembodiments, the conductive material comprises a biocompatible metal,such as gold, carbon, or aluminum. In the embodiment shown, theelectrodes 116, 118, 120 and 153-164 are embedded in a substrate 114such that adjacent electrodes are separated from each other by portionsof the substrate 114, for example, in a laminate configuration that isperpendicular to the vertical axis 110 of the microwell (see FIG. 2).

3. Microchannel

As shown, for example in FIGS. 1, 2, and 3, in an inverted openmicrowell system 100, 101, the microwell 102 communicates with amicrochannel 104 for carrying a fluid 130 that may contain one or moreparticles 128. The microchannel 104 can be associated with one or aplurality of microwells 102, for example, with one or more rows ofmicrowells 102 in the device 100, 101.

The microchannel can be formed of a polymer, for example, polyimide,closed with a top cover 124, preferably formed of a clear material suchas glass or suitable plastic, such as PMMA, polycarbonate, PEN, PET,cyclo-olefin polymer. The formed microchannel can then be adhered to thetop of the microwell(s), for example, with a biocompatible adhesive.

The upper end 106 of the microwell is open to the microchannel 104. Forexample, fluid 130 may be caused to flow through the microchannel 104 bycreating a pressure differential at each end of the microchannel 104.The microchannel 104 may be connected to a pump 132 such as aperistaltic pump, for example, or a source of pressurized air, and thelike. The flow rate of fluid 130 in the microchannel can be controlledby adjusting the pressure differential between the ends of themicrochannel 104, thereby controlling the rate at which particle(s) 128pass by the microwell(s) 102.

4. Plurality or Array of Microwells

As shown in FIG. 3, the device 101 may include a plurality of microwells102 arranged in an ordered pattern. For example, the device 101 mayinclude an array or matrix of microwells, for example in rows andcolumns that may be in communication with one or a plurality ofmicrochannels 104. The device 101 can be used to execute multipleparallel operations on a plurality of microwells 102 simultaneously.While the discussion below relates to a single microwell 102, theembodiments described are applicable to each of a plurality ofmicrowells 102 for example, that may be disposed as an array or matrixof microwells 102 in a device 101, and may contain variousconfigurations of microwells 102 and microchannels 104. Theconfiguration of microwells can be designed for convenience, for exampleto accommodate a traditional system for transfer, such as a 96-wellmicrotiter plate or 1536 well micro-vial assembly, and the like. Thedesign may also be for specific purpose, such as high throughput.

5. Producing an Inverted Microwell Device

To form an inverted microwell device, in one example, microwells 102 canbe produced as through holes formed through a substrate 114 withembedded electrodes 116, 118, 120. The through holes can be formed byconventional boring methods using, for example, mechanical or lasertechniques. Exemplary production of an inverted microwell system isdescribed in the Examples below.

In some embodiments, the microwells 102 have a circular or roundedcross-section, however the microwells 102 can have other cross-sectionalshapes, for example, hexagonal, rectangular, square, conical, and thelike shapes. The diameter of the microwell 102 is generally less than1000 micrometers and may vary, for example, according to the dimensionsof cells or other particles to be deposited in the microwell, as well asaccording to geometrical relationships needed to retain sufficientsurface tension to form and maintain the meniscus 122. The diameter ofthe microwell can be, for example from about 70 μm to about 150 μm, or,for example, from about 50 μm to about 100 μm. The depth (height) of themicrowell is preferably the same or greater than the diameter, and canbe, for example, from about 50 μm to about 300 μm, for example fromabout 70 μm to 200 μm. The ratio of diameter to depth (height) can be,for example, about 1 to 1; 1 to 1.25; 1 to 1.5; or 1 to 2.

In one embodiment a device was constructed where the microchannel had alength of 27 mm, a width of 350 μm and a height of 150 μm. The microwellhad a diameter of 100 μm and a height of about 75 μm. No hydrophobic orhydrophilic coatings were applied. The polyimide surfaces in themicrochannel and on the bottom side of the device had a modesthydrophilic behavior as did the top side of the channel constructed withtransparent polycarbonate, while the inner part of the mechanicallydrilled microwell, had an increased roughness that consequentlyincreased the hydrophilic behavior of the inner part of the microwell.This difference in surface properties, where the inner part of themicrowell was more hydrophilic than the microchannel and the bottomsurface, combined with the proper sizing of the microchannel and themicrowell, was surprisingly sufficient to allow the microwell to beproperly filled with fluid from the microchannel, without producing anyleakage from the bottom of the open microwell.

6. Hydrophilic/Hydrophobic Surfaces

To facilitate introduction and retention of one or more particle(s) 128in each microwell 102, the device 100, 101 may be configured such thatcertain surfaces are hydrophobic while other surfaces are hydrophilic.In some embodiments, the surfaces are made hydrophobic or hydrophilic byapplying a coating to the surfaces. For example, in embodiments thatutilize an aqueous fluid, the bottom surface of the microchannel 143 andvertical wall of the microwell 112 may be hydrophilic and the lowersurface 142 may be hydrophobic. As another example, in embodiments thatutilize a lipidic fluid, the bottom surface of the microchannel 143 andvertical wall of the microwell 112 may be hydrophobic and the lowersurface 142 may be hydrophilic. Kapton and Polyimide present a discretehydrophilic behavior, with a contact angle in the range between 20 and70 degrees. By applying a plasma surface treatment, for example anoxygen plasma, Kapton and Polyimide surfaces become more hydrophilic.Hydrophobic coatings which can be applied to Polyimide or Kaptonsurfaces include FEP, Certonal® FC-732 or Chemlease® 41-90.

Fluid 130, for example, a physiological buffer or culture medium, is fedinto a microchannel 104 positioned above and in fluid communication withthe upper open end 106 of one or more microwell 102. The micro-size ofthe well and hydrophobic behavior of the lower open end 108 of themicrowell 102 in contrast with a hydrophilic nature of the microchannel104 and upper end of the microwell 102, permit fluid to fill themicrowell having an open bottom, without leakage from the lower open end108. When using microwell with a closed bottom, to facilitate filling ofthe microwell, a wetting agent, for example ethanol or ethanol mixedwith water, can be initially inserted in the microchannel prior toinserting a physiological medium is. Capillary action and surfacetension retain the fluid 130 in the microwell, and form a meniscus 122at the fluid-air interface, as shown in FIG. 1. In an embodiment, thefluid may contain a wetting agent to assist filling of the microchanneland/or microwell. Exemplary wetting agents include, for example ethanol,Tween-20, and SDS. Prior to insertion of the cell suspension,microchannels are properly rinsed with physiological medium, forexample, phosphate buffer saline (PBS), to remove residuals of thewetting agents.

7. Controlled Delivery and Focusing of Particles

7.1 Controlled Delivery

A fluid containing particles, for example, cells, can be delivered tothe microchannel 104 at a concentration and flow rate designed to permita limited number of particles, for example a single particle, to bedistributed into each microwell 102. Electrodes can be used to transportparticles in the microchannel 104 to permit or deny access of particles128 from the microchannel 104 into the microwell 102, to manipulate aparticle(s) 128 within the microwell, to focus and hold a particle in adesired position within the microwell, to induce structural orfunctional changes in a particle or particles disposed within themicrowell, and/or to sense and/or measure the presence, movement, orchange in a particle within the microwell, or to sense and/or measurethe presence of molecules produced by a particle within the microwell.

An embodiment, for example, described by FIGS. 9-11, exploits the fluidflow in the channel to transport the particles of interest from an inletto a particular microwell, with the additional possibility to configuredielectrophoretic forces so that the particle can skip a microwell andreach another along the microchannel. An embodiment, described by FIGS.12 and 13, shows an array of electrodes that does not require fluid flowto transport a particle in the microchannel toward a microwell, butapplies dielectrophoretic forces to do so.

7.2 Transport of Particles to Microchannels in the Absence of Fluid Flow

Precise transport in microchannels for precise delivery to specificmicrowells is needed when a precise location for each particle isdesired. One way to control precise delivery is based on a sequence ofchanges of electrical parameters that provides very precise resultswithout relying on fluid flow or similar techniques.

A particle having a complex dielectric constant, when exposed tospatially varying electric fields, is pushed by negativedielectrophoresis according to the value of the Clausius-Mossottifactor. In this case, the particle will move toward the minimum of theelectric field and will be repelled by the maximum of the electricfield.

As shown in FIGS. 10, 11, 12 and 13, for example, electrodes 155 and 156are positioned above the electrodes 153, 154, 157, 158, 159, 160, 161,and 162. In particular, electrode 155 is positioned above electrodes153, 157, 159, and 161, while electrode 156 is positioned aboveelectrodes 154, 158, 160, and 162. These electrodes can be used tocreate a pattern of electric field strength to cause the particle ()128 to move in a desired direction in the microchannel 104.

In one example, each of the electrodes 153, 157, 161, and 156 isconnected to a sinusoidal voltage source at a specified electricalpotential. Facing electrode partners 154, 158, 162, and 155 is eachconnected to a sinusoidal voltage source of the same amplitude as itsfacing partner, but with a phase difference of 180 degrees shift.Electrodes 159 and 160 are connected to ground (G1).

In this scenario, the electric field in the microchannel 104 reaches aminimum (Δ) 170 in the spacing between electrodes 159 and 160 as theyare at the same electric potential and their thickness creates a recessthat induces a strong minimum of the electric field. The electric fieldin the microchannel 104 reaches a maximum where electrode pairs 153-154,157-158, 161-162 facing each other are polarized with voltages with aphase difference of 180 degrees. In this case, a particle () 128 istrapped in the position where the minimum (Δ) 170 between electrodes159, 160 is surrounded by locations where the electric field reaches amaximum of intensity.

The location of the trapped particle can be changed and moved, forexample, as shown in FIG. 13, toward left in two phases. During a firstphase, the voltage of electrodes 157, 158 located to the left of theexisting resting position is set to ground (G2). This creates a minimum(Δ) electric field that is of the same strength as that located betweenelectrodes 159 and 160. During a second phase, electrode 159 ispolarized as electrode 161 and electrode 160 as electrode 162. Thispolarization creates a strong electric field between electrodes 159 and160. This field pushes out a particle trapped there that will fall inthe closest minimum (Δ) of electric field, moving the particle onelocation to its left.

Hence, the displacement of the particles obtained in this way does notrequire a fluid flow.

In another example, electrodes 155 and 156 are substituted by a uniqueelectrode 190 positioned along the top side of the microchannel as shownin FIG. 14. The electrode 190 can be implemented either by closing themicrochannel on top with a metallic material or with a transparentmaterial, such as glass or a transparent plastic, coated with IndiumThin Oxide (ITO). This conductive coating provides an electricalconnection while keeping a transparent top cover that does not preventoptical inspection or lighting from top side.

In this example, the electrode 190 is connected to ground, while each ofthe electrodes 153, 157, and 161 is connected to a sinusoidal voltagesource at a specified electrical potential. Facing electrode partners154, 158, and 162 is each connected to a sinusoidal voltage source ofthe same amplitude as its facing partner, but with a phase difference of180 degrees. Electrodes 159 and 160 are connected to a sinusoidalvoltage source of the same amplitude as the electrodes 153, 157, and161, but with a phase difference of 90 or 270 degrees.

In this scenario, the electric field in the microchannel 104 reaches aminimum (Δ) 170 in the spacing between electrodes 159 and 160 as theyare at the same electric potential and their thickness creates a recessthat induces a strong minimum of the electric field. The electric fieldin the microchannel 104 reaches a maximum where electrode pairs 153-154,157-158, 161-162 facing each other are polarized with voltages with aphase difference of 180 degrees. In this case, a particle () 128 istrapped in the position where the minimum (Δ) 170 between electrodes159, 160 is surrounded by locations where the electric field reaches amaximum of intensity.

With this electrode configuration, the location of the trapped particlecan be changed and moved, for example, toward left in two phases. Duringa first phase, the voltage of electrodes 157, 158 located to the left ofthe existing resting position is set to a sinusoidal voltage source ofthe same amplitude as the electrodes 153, 157, and 161, but with a phasedifference of 90 or 270 degrees. This creates a minimum (Δ) electricfield that is of the same strength as that located between electrodes159 and 160. During a second phase, electrode 159 is polarized aselectrode 161 and electrode 160 as electrode 162. This polarizationcreates a strong electric field between electrodes 159 and 160. Thisfield pushes out a particle trapped there that will fall in the closestminimum (Δ) of electric field, moving the particle one location to itsleft.

FIG. 12 shows a structure that can be used to precisely deliverparticles in a microwell 102 in absence of fluid flow. Electrode pair155, 156 is not shown in FIG. 12, but as shown in FIGS. 9 and 10, thepair is located above the other electrodes, with electrode 155 in aposition above electrodes 163, 116, 153, while electrode 156 is in aposition above electrodes 164, 116A, 154.

If electrodes 116 and 116A are polarized at the voltage of 156 and 155respectively, while electrodes 153 and 154 are at the same groundvoltage, a particle can be trapped by negative dielectrophoresis in thelocation (Δ) between 153 and 154. Connecting then to ground electrodes116 and 116A while 153 and 154 are polarized as 156 and 155, a particleis pushed toward the minimum (Δ) 170 of electric field between 116 and116A where the microwell is located.

Since electrode 116, 116A surrounds the microwell 102 and is connectedto ground, an electric field is induced in the channel beneath themicrowell 102 due to the presence of electrodes 155 and 156. Followingthe previous analysis, the particle is now pushed toward the microwell.

In case the location where the particle descent in the microwell must becontrolled in a precise way, electrode 118 can be polarized connectingit to a voltage source having amplitude substantially smaller than thatused to control electrodes 155 and 156 and with a phase rotated 90 or270 degrees with respect to electrodes 155 and 156. This polarizationcreates a dielectrophoretic force in the microwell that pushes theparticle toward the center of the microwell.

If the delivery of the particle in the microwell must be avoided, adifferent polarization of the electrodes is recommended in absence offluid flow. In particular, assuming that a particle is trapped betweenelectrodes 153, 154 that are connected to ground, the delivery can beavoided by connecting to ground electrodes 116, 163, 164 while electrode116A is polarized as electrode 155 and electrode 153 is polarized aselectrode 156 and electrode 154 is polarized as 155. These polarizationscreate a temporary displacement of the particle from the locationbetween 153 and 154 to a location toward electrode 116 (and not thelocation between 116 and 116A) and finally to a stable delivery positionbetween electrodes 163 and 164 where a minimum of the electric field islocated.

In another example, the structure reported in FIG. 12 can be used toprecisely deliver particles in a microwell 102 in absence of fluid flow.Electrode pair 155, 156 is substituted by an electrode 190 positioned ontop side of the microchannel, as shown in FIG. 16, and connected toground.

If electrodes 116 and 116A are connected to a sinusoidal voltage sourceof the same amplitude and a phase of 0 degrees and 180 degrees,respectively, while electrodes 153 and 154 are connected to the samesinusoidal voltage with a phase shift of 90 or 270 degrees, a particlecan be trapped by negative dielectrophoresis in the location (Δ) between153 and 154. Connecting then the electrodes 116 and 116A to a sinusoidalvoltage with a phase shift of 90 or 270 degrees while 153 and 154 areconnected to a sinusoidal voltage of the same amplitude and a phase of 0degrees and 180 degrees, respectively, a particle is pushed toward theminimum (Δ) 170 of electric field between 116 and 116A where themicrowell is located.

Since electrodes 116, 116A surround the microwell 102 and are connectedto the same voltage, an electric field is induced in the channel beneaththe microwell 102 due to the presence of electrode 190. Following theprevious analysis, the particle is now pushed toward the microwell.

In case the location where the particle descent in the microwell must becontrolled in a precise way, electrode 118 can be polarized connectingit to a voltage source having amplitude substantially smaller than thatused to control electrodes 116 and 116A and with a phase rotated 90 or270 degrees with respect to electrodes 116 and 116A. This polarizationcreates a dielectrophoretic force in the microwell that pushes theparticle toward the center of the microwell.

If the delivery of the particle in the microwell must be avoided, adifferent polarization of the electrodes is recommended in absence offluid flow. In particular, assuming that a particle is trapped betweenelectrodes 153, 154 that are connected to the same sinusoidal voltagewith a phase shift of 90 or 270 degrees, the delivery can be avoided byconnecting electrodes 116, 163, 164 to ground while electrodes 153, 116Aand 154 are connected to the same sinusoidal voltage with a phase 0, 90and 180 degrees, respectively. Electrode 190 always remains connected toground. These polarizations create a temporary displacement of theparticle from the location between 153 and 154 to a location towardelectrode 116 (and not the location between 116 and 116A). Finally, byconnecting electrode 116 to the same sinusoidal voltage of electrode 153with a phase shift of 270 degrees, the particle reaches a stabledelivery position between electrodes 163 and 164 where a minimum of theelectric field is located.

These structures with the voltages proposed do not rely on the presenceof a flow of fluid for the displacement and delivery of particles. Thefluid flow is however appropriate to provide a suitable cooling of thestructure. These structures can be used to move single cells along amicrochannel and drive them into an open microwell or prevent them fromentering into an open microwell by properly applying a sequence ofpolarization signals.

7.3 Transport of Particles in Microchannels and Open Microwells in thePresence of Fluid Flow

FIGS. 9-11 diagram the structure of a microwell 102 in fluidcommunication with a microchannel 104 closed on top by a surface 124covering liquid contained in the microchannel. Particles such as cellsor beads flowing in the channel fluid such are subjected toelectromagnetic forces causing a particle 128 to be pulled toward amicrowell 102 or pushed away. This structure is specifically suited tooperate when there is an appreciable fluid flow in the channel that cantransport the particles. See, for example, Faenza et al., 2011 (May),“Controlled isolation and patterning of k562 leukemia cells usingelectrically activated microchannels”, In: International Conference onMicrotechnologies in Medicine and Biology.

Electrodes are built into the structure of the microwell andmicrochannel to control particle movement. For example, FIG. 9 shows amicrowell 102 surrounded by an electrode 116 shaped so that it presentsan opening toward the direction of the flow in a direction that is fromright to left in FIG. 9.

The specific embodiments useful to control particle movement that areshown in these Figures include those electrodes 116, 118 positioned inthe microwell 102, and those electrodes 153, 154 positioned in themicrochannel 104. FIG. 10 shows a microwell 102 in communication with amicrochannel 104, each having a vertical wall 112 formed by a laminatestructure containing a dielectric substrate 114 and electrodes.Microchannel electrodes 153-164 are disposed in the microchannel 104while electrode pairs 153-154 and 155-156 positioned near the junctionof the microchannel with the microwell 102 at its upper open end 108.Microwell electrodes 116 and 118 are disposed near the upper end andlower ends of the microwell, respectively.

A particle () 128 moving in the microchannel 104 with the fluid flow issubject to a dielectrophoretic force if its complex dielectric constantis different from that of the surrounding fluid 130. Particles ofinterest, such as cells, microbeads or liposomes, are characterized bybeing driven by negative dielectrophoresis that attracts these particlestowards the minima (Δ) 170 of the electric field. The particles alsohave a relative density higher than that of the surrounding buffer. Forthis reason, they fall under the effect of gravity.

FIGS. 10 and 11 show that a particle placed in the flow of themicrochannel 104 encounters electrodes 153, 154, 155, 156 beforereaching the microwell and electrode 116. Various configurations ofapplied voltages can cause a particle 128 to be trapped or ejected by anelectric field, by principles of dielectrophoresis, where a particlemoves in a direction toward a minima (Δ) 170 of an electrical field andaway from an area with a strong electrical field.

In an example, electrode 116 is set to ground. Electrodes 155 and 156are polarized by an external power source to a voltage generator. Thevoltage of electrode 155 has a phase that is rotated of 180 degreesversus that of electrode 156. Voltages applied to the electrodes 155 and156 have the same amplitude. The voltages could change as sinusoidalsignals or a square waves. For this reason, there is a relatively largeelectric field between the two electrodes 155 and 156.

The polarization of electrodes 153 and 154 will vary with the task oftrapping or ejecting a particle. More specifically, electrodes 153 and154 can be connected to ground in a configuration that is called“trapping”. Alternatively, electrodes 153 and 154 can be polarized withsinusoidal voltages or square waves. When they are polarized, the phaseof electrode 153 will be the same of that of electrode 156, whileelectrode 154 will have the phase of electrode 155. This secondconfiguration is called “ejecting”. Refer to FIG. 11 for the relativelocation of the electrodes.

Following the electrode configuration shown in FIGS. 9-11, when adesired particle 128 flowing in the microchannel 104 is to be trapped ina microwell 102, electrodes 153 and 154 are set at the ground potential.Electrodes 155 and 156 are polarized by an external power source to avoltage generator. The voltage of electrode 155 has a phase that isrotated of 180 degrees versus that of electrode 156. Voltages applied tothe electrodes 155 and 156 have the same amplitude. The voltages couldchange as sinusoidal signals or a square waves. For this reason, thereis a relatively large electric field between the two electrodes 155 and156.

In this condition, a “trapping” situation exists, since the electricfield reaches a strong minimum in the location 170 placed betweenelectrodes 153 and 154. The remaining region in the channel is subjectto a stronger electric field that pushes the particle towards the minima(Δ). The thickness of electrodes 153 and 154 as well as all otherelectrodes is about 5 to 30 microns. This creates a region (Δ) 170 wherethe field reaches a minimum, and where the particles have approximatelythe same size of the surrounding electrodes.

Region (Δ) 170 is aligned to the opening of the microwell 102. Aparticle trapped at the minima region (Δ) 170 is pushed by the fluidflowing in the microchannel toward the opening of the microwell along ahorizontal path. The distance between this region (Δ) 170 and theopening of the microwell 102 is kept to a value as small as feasiblegiven the manufacturing technology. If a standard Printed Circuit Boardtechnology is used, a distance of about 20 to 100 microns is typical.Since electrode 116 surrounds the microwell 102 and is connected toground, an electric field is induced in the channel beneath themicrowell 102 due to the presence of electrodes 155 and 156. Followingthe previous analysis, the particle is now pushed toward the microwell.

In case the location where the particle descent in the microwell must becontrolled in a precise way, electrode 118 can be polarized connectingit to a voltage source having amplitude substantially smaller than thatused to control electrodes 155 and 156 and with a phase rotated 90 or270 degrees with respect to electrodes 155 and 156. This polarizationcreates a dielectrophoretic force in the microwell that pushes theparticle toward the center of the microwell.

If a particle flows in the microchannel 104 and it is not intended formicrowell 102, the desired effect can be achieved by polarizing theelectrodes 153 and 154 in the “ejecting” configuration. In this case,the minimum of the electric field is placed in the middle of themicrochannel 104. In this location, the particle is exposed to the fluidflow and is not trapped by the microwell 102 if the distance between themicrowell 102 and electrodes 153 and 154 is small.

In another example the controlled movement of the particle can beobtained by initially trapping a particle between electrodes 153 and 154and then programming the “loading” or “ejecting” configuration byproperly setting the polarization of electrodes 153, 154 and 116 whilekeeping electrode 155 constantly connected at a sinusoidal voltage andelectrode 156 connected at the same sinusoidal voltage of electrode 155shifted by 180 degrees. In order to initially trap the particle,electrodes 153 and 154 are tied to ground, and electrode 116 isconnected to the same sinusoidal voltage of electrode 155 shifted by 90degrees. By setting a fluid flow directed from electrodes 153 and 154towards the microwell, it is possible to find a value of the flow whichis low enough for trapping a particle in the minimum of the electricfield located between electrodes 153 and 154 with a drag force which isnot sufficient for moving forward the particle and contrast the electricfield created between electrode 116 on one side and electrodes 155 and156 on the other side. If the particle needs to be delivered to themicrowell, then the electrode 116 is connected to ground and the dragforce will move the particle towards the microwell and then into themicrowell. If, instead, the particle has to move forward, bypassing themicrowell, electrodes 153 and 116 are connected at the same sinusoidalsignal of electrode 156, while electrode 154 is connected at the samesinusoidal voltage of electrode 155. In this configuration, the particlefirst moves upwards and then towards the portion of electrode 116 underelectrode 156, where a minimum of the electric field is located. In thisway the microwell is circumvented and the particle is not delivered.Assuming now to have a second microwell following along the channel,with the same architecture illustrated in FIG. 9, a new set ofelectrodes 153 and 154 can be set to ground and the electrode 116 of thefirst microwell can return to the initial voltage. In this way theminimum is now located between the couple 153, 154 of the secondmicrowell and the particle will position between these electrodes. Thisscheme can be repeated for several wells positioned along the samemicrochannel.

7.4 Detection of Particles in a Microchannel

When a particle () in a microchannel 104 is manipulated by theelectrodes shown in FIGS. 9-14 in such configurations allowing theparticle to be delivered into a target microwell, the electrodes producethe effect of aligning the particle along the axis F-F′ as shown inFIGS. 15-16. Moreover the particle is vertically pushed onto the bottomside of the microchannel. The precise positioning of particles along apredefined axis and at a precise vertical location make them suitable tobe detected either optically or by impedance measurement, assuming acontinuous laminar fluid flow is present in the microchannels and withthe effect of dragging the particle along the F-F′ direction. Thebenefit of implementing cell focusing through dielectrophoresis toimprove optical detection and cell delivery in microwell isdemonstrated, for example, in Duqi et al., 2011 (May), “Automatedisolation of a programmable number of cells into microwells using DEPforces and optical detection”, International Conference onMicrotechnologies in Medicine and Biology.

In one embodiment, a structure composed of four electrodes 170, 171,172, 173 is properly positioned downstream to the manipulationelectrodes 153, 154 as shown in FIG. 15. Electrodes are designed as toallow the particle moving along the F-F′ direction to pass through thegap between electrodes 170-171 and 172-173. Electrodes 171, 173 areconnected to an excitation signal 188 represented by an alternatevoltage, while electrodes 170, 172 are connected to a readout circuit186 which amplifies the output currents 180 and 182 and provides a finaloutput signal 184 proportional to the difference or the ratio betweensignals 180, 182. This is accomplished by using known circuit schemes.The presence of upstream electrodes 153, 154, 155, 156 has the effect ofaligning cells along the axis where the signal-to-noise ratio producedon the output signal 184 by the passage of single cells is maximum.

In another embodiment, the detection of a single cell after properpositioning and alignment performed by electrodes 153, 154, 155, 156, isperformed by means of a structure composed of three electrodes 174, 175,176, as shown in FIG. 16. Electrode 175 is connected to an excitationsignal 188 represented by an alternate voltage, while electrodes 174,176 are connected to a readout circuit 186 which amplifies the outputcurrents 180 and 182 and provides a final output signal 184 proportionalto the difference or the ratio between signals 180, 182. The presence ofupstream electrodes 153, 154, 155, 156 has the effect of aligning cellsalong the axis where the signal-to-noise ratio produced on the outputsignal 184 by the passage of single cells is maximum.

In one embodiment the detection of single cells after proper positioningand alignment performed by electrodes 153, 154, 155, 156 is performed byoptical detection, either under fluorescence or normal lightingconditions, by positioning an optical excitation and detection system ontop or bottom side of the microwell array and by aligning the opticalsystem along the axis F-F′.

Once the particle is detected according to one of the methods previouslydescribed, in a specific embodiment the structure represented byelectrodes 116, 153, 154, 155, 156, 116, 118 and the microwell 102 andreported in FIGS. 9-11 is duplicated and positioned downstream of thedetection region or electrodes. The control of these electrodes isdependent on the sensing signal 184 obtained by optical or electricalmeasurement, in such a way that a single cell, when detected, can beselectively pulled into a target microwell or pushed away from it. Thisembodiment enable the precise positioning of single cells or apredetermined number of cells in microwells.

7.5 Focusing of Particles in a Microwell

When a particle 128 enters a microwell 102, gravity causes the particle128 to descend through the microwell 102 toward the meniscus 122. Toassure that the particle 128 remains substantially centered in themicrowell 102, microwell electrodes 116, 118, 120 may be powered toforce the particle 128 to a central position across the diameter of themicrowell 102. For example, the voltages applied to the electrodes 116,118, 120 may be controlled to generate a substantially zero orsubstantially vertical electric field at or near the vertical axis ofsymmetry of the microwell 102. This results in the particle 128 beingforced to the center of the microwell 102, causing the particle 128 todescend the length of the microwell along a central axis, depositing ator near the center of the meniscus 122. Descent of the particle may befacilitated by gravity or by electromagnetic forces, or a combination ofthese.

While the microwell electrodes 116, 118, 120 are shown as annularelectrodes, in some embodiments the microwell can include pairs ofelectrodes, for example, facing electrodes, to effect particlemanipulation and movement within the microwell. In addition, themicrowell may contain additional electrodes, including for example,electrodes useful in a particular analysis or reaction, such as sensingand/or measuring electrodes for sensing particles, products of cells,and cellular changes. Sensing or measurement electrodes can be used, forexample, to measure changes in impedance, optical signal, polarity, andthe like electrical signals

A particle 128 deposited on the meniscus 122 remains stably positionedon the meniscus 122 even after electromagnetic forces, for example,voltages applied to the electrodes are suspended. Exemplary approachesto applying voltages to the electrodes to manipulate particles withdielectrophoresis are described in published U.S. Patent Application No.2009/0288963, incorporated herein by reference in its entirety.

When a particle 128 has entered a microwell 102, electrodes 153 and 154positioned on the bottom side of the microchannel near the junctionbetween the microwell 102 and the microchannel 104 may be powered toprevent additional particles flowing in the microchannel from enteringthe microwell. For example, the phase of the voltage applied to theelectrodes 153 and 154 can be controlled to repel other particles in thefluid, essentially closing the microwell 102. This may occur after theparticle has dropped past the electrode 116, for example, whileelectrodes 118 and 120 may be employed to center the particle in themicrowell, permitting the particle to descend the length of themicrowell by gravity, and remaining substantially in a central position,even in the absence of applied electromagnetic force.

In some embodiments, two or more particles 128 are permitted to enterthe microwell 102 and are positioned so as to force the particles intocontact or into close proximity with each other. For example, after afirst particle 128 has been positioned on the meniscus 122, the voltageapplied to the electrodes 153 and 154 may be removed, opening themicrowell 102 for receipt of one or more additional particle from themicrochannel 104. When one or more additional particle(s) enter themicrowell 102 from the microchannel 104, the additional particle(s) 128may be centered in the microwell 102 by applying voltages to electrodes118, 120 as discussed above. The additional particle(s) 128 descendsalong a vertical axis of symmetry 110 of the microwell until theadditional particle(s) is positioned in contact or in close proximity tothe particle 128 on the meniscus 122. In alternative embodiments, allparticles are introduced into the microwell at substantially the sametime, and the electrodes 116, 118, 120 are powered to focus theparticles to the center of the microwell as an aggregate group of cells,forming a cluster of deposited cells at the meniscus.

8. Observing Deposited Cells Microscopically

Because of the open nature of the microwells 102, it is possible toobserve the docking of a particle 128 on the meniscus 122 at thefluid/air interface at the lower end 108 of the microwell 102, forexample, using a microscope, camera, or other optical device. Theparticle(s) 128 can be viewed from either the upper end 106 of themicrowell 102, for example through the microchannel 104 and a top cover124 that may be fabricated with a transparent material such as glass orplastic, or viewed from the lower end 108 of the microwell, for example,where the view is not impeded.

9. Stable Retention at the Meniscus

Surprisingly, particles, including living cells, are stably retained onthe meniscus when the electric field is deactivated, and while fluid iscontinuously flowing in the microchannel and thereby into themicrowells. It has been surprisingly found that cells disposed on themeniscus are not dislodged by fluid washes or changes in media in theabsence of an electric field, assuming a proper sizing of the microwell.This stable retention permits removal of harmful fluids, for examplebuffers and reagents needed for DEP manipulations, and permitsreplacement with fresh buffers and culture media more suitable formaintenance and vitality of deposited biological particles and/or moresuitable for particular analytical procedures within the microwell.

Stable retention on the meniscus can be achieved if the shear stressinduced by the horizontal component of the fluid velocity is relativelylow or null at the bottom level of the microwell. By changing therelative ratio between the diameter and depth of closed microwells thehorizontal component of the fluid velocity changes as well, as explainedin Han et al., 2010, “Integration of single oocyte trapping, in vitrofertilization and embryo culture in a microwell-structured microfluidicdevice,” Lab on a Chip, vol. 10, no. 21, pp. 2848-2854. A similarbehavior is observed in inverted open microwells where the bottom sidefeatures a meniscus between the air and the fluid.

In one embodiment, the microwell is sized as to ensure a depth equal orhigher than the diameter. For instance, a depth of 100 microns or morefor a diameter of 75 microns is considered. This microwell sizing allowsto maintain a horizontal fluid velocity of less than 1 micron/sec for anaverage fluid speed of 2.5 mm/sec in the microchannel.

A continuous supply of nutrients in buffer or media flowing in themicrochannel, as well as removal of catabolites produced by the cells ispossible in the inverted open microwell system, and helps to maintainthe viability of the cells or other particles disposed in the microwell.In addition, exposure to the atmosphere outside the device at the lowerend open of the microwell permits exchange of gases, contributing tomaintaining the health and viability of the particle or particlescontained in the open microwell.

Surprisingly, in the inverted open microwell system, a combination offeatures, including small volume, stable retention, efficient buffer andmedia exchange, efficient gas exchange, and the ability to manipulateand focus cells to a desired position with minimum use ofelectromagnetic forces, permits not only efficient use of the system toevaluate characteristics of single cells, but also to retain, reuse, andrecover the original cells for clonal expansion.

10. Recovery of Microwell Contents

In an embodiment, the contents of the microwell can be recovered, forexample, by disrupting the surface tension at the meniscus. Disruptionof surface tension can be accomplished, for example, by applying apressure pulse to the microchannel 104, for example, from a pump 132 ora source of pressurized air, and the like, with a pressure ranging from0.5 bars to 3 bars and a pulse duration ranging from 1 millisecond to100 milliseconds. The contents of the microwell are released, forexample, as a droplet 138 containing, for example, a single particle,cell, or cluster of particles or cells, in fluid of the microwell, intoa suitable receptacle 126, for example a reservoir, microtiter plate,collection vial system, capture surface, and the like, positioned underthe microwell 102 (FIG. 4, 5). In an embodiment, the receptacle 126 caninclude a filter or membrane for separating particle(s) from fluidrecovered from the microwell 102.

In an embodiment, fluid surrounding a particle 128 in the microwell canbe concentrated by evaporation prior to recovery from the microwell. Forexample, replacing the fluid in the microchannel with air or suitablegas can result in reduced volume in the microwell, as evaporation willoccur at the lower end 108 of the microwell open to air or gas. Applyingpressure to the air/gas in the microchannel can cause release of theconcentrated fluid in the microwell to be released into thereceptacle(s).

In one embodiment, the microwell contains a semipermeable membrane 134at or near the air/fluid interface at the lower end of the microwell,the membrane retaining the particle(s). In this embodiment, fluid can berecovered from each microwell 102 into the receptacle(s) 126.

Where the device 101 contains a plurality of microwells, the content ofmultiple wells can be transferred to a matched set of multiplereceptacles in parallel, for example to multiple wells of a microtiterplate.

During transfer of a cell from the microwell, it may be possible thatother cells or particles are trapped in the microchannel and may betransferred along with the cell(s) in the microwell when the pressurepulse is applied. To limit this problem, a bovine serum albumin (BSA)coating or other anti-stiction coating, for example, an organicpassivation layer such as a fluorinated fatty acid self-assembledmonolayer (SAM) or alkylhalosilane can be added to the microchannelsprior to use.

EXAMPLES

The invention may be more fully understood by reference to the followingExamples. The examples are meant to describe specific exemplaryembodiments of the invention and are not meant to limit the scope of theinvention.

Example 1 Analysis of Fabricated Microchannel and Microwell Geometries1.1 Fabrication

A device containing a plurality of open microwells was created bydrilling through holes through a multilayer flexible printed circuitboard (PCB) substrate. The dimensions of the microwells were varied toanalyze the effectiveness of specific geometries of the microwells. Inthis study, the diameter of the drilled holes ranged from 70 μm to 150μm, PCB thickness was between 75 μm and 350 μm, and each device includeda matrix of 6×6 or 8×8 holes. Preferred implementations were prepared asshown below in Table 1 in order to directly interface the open microwellarray to standard microtiter plates and easily perform the recovery andtransfer of cells into microtiter wells.

Microchannels 104 having a thickness ranging from 30 μm to 200 μm and awidth ranging from 200 μm to 1 mm were fabricated of polyimide, on thetop side of the plurality of microwells 102, disposed above the upperend 106 of the microwells. A channel cover 124 was formed of transparentpolycarbonate, having a thickness of 750 μm by adhering it atop themicrochannel.

TABLE 1 Microwell arrays features Number of Rows × Well-to-well andchannel- microwells Columns to-channel pitch 24 6 × 4  18 mm 96 12 × 8   9 mm 384 24 × 16 4.5 mm 1536 48 × 32 2.25 mm 

Microchannel walls were fabricated in polyimide and bonded to a topcover using an adhesive properly cured at a temperature of 70° C. for 2hours until overnight, in order to ensure the biocompatibility of theadhesive, or using an adhesive laminated at room temperature and thencoated with BSA 1 mM to ensure biocompatibility.

In an alternative embodiment, a photosensitive polymer film (OrdylSY550, Elga Europe) having a thickness of 55 μm was attached to a glasstop cover and structured to create the microchannel, before attaching itto the underlying flexible PCB containing the drilled microwells. Holeswere formed in the top cover to provide input and output fluidicconnections or embedded fluid reservoirs. The holes had a diameter ofabout 0.45 mm.

Fluid or fluid containing cells and particles were inserted in eachmicrochannel using a peristaltic pump (Watson Marlow 101U/R) connectedas shown in FIG. 5 in one study, and using a syringe pump connected tothe microchannel inlet (KDS-210, KD Scientific, Holliston, Mass.) inanother. Fluid flow ranged from 1 microliter/hour to 20microliters/minute, depending on the channel section and the specificoperation to be executed on the particles. Fluid leakage from the lowerend of the microwells was prevented by the capillary forces acting inthe microwells. In some cases a hydrophobic coating (Certonal® FC-732)was added on the bottom surface of the device to increase the resistanceto fluid leakage. As a result, the fluidic system provides fluid in themicrochannel that surprisingly fills the microwells without leaking fromthe lower end.

1.2 Analysis of Cells Deposited by Gravity

Live K562 cells (immortalized human myelogenous leukemia cells) weresuspended in physiological solution (NaCl 0.9% w/v or PBS) at aconcentration of 1.6×10e6 cells/milliliter and inserted into amicrochannel of an inverted microwell device using a peristaltic pump.The microchannel width was 600 μm and the height was 55 μm, and eachmicrowell had a diameter of 70 μm.

The input pump was set to operate in cycles where each cycle wascomposed of two phases. During Phase 1 the fluid was active for 1 minuteat a speed of 9 microliters per minute, while during Phase 2 the pumpwas deactivated for a period of 1 minute and 30 seconds. During Phase 1cells flowed in the channel along random trajectories. At the beginningof Phase 2 cells were stationary in the channel in random locations.During Phase 2 a certain number of cells were allowed to enter into eachmicrowell by sedimentation.

Using an inverted microscope, the content of each well was checked andthe possible presence of a cell in the microwell as a consequence ofsedimentation was detected. The results demonstrated that it is possibleto control the distribution of cells entering into the microwells byadjusting the cell concentration. In the condition here described weobtained the distribution reported in Table 2.

TABLE 2 Loading efficiency of cells in microwells (total of 41 samples)Number of cells loaded Frequency 0 51% 1 42%  2+ 7%

The result of the load phase demonstrated that a single cell or multiplecells can be deposited in a microwell reaching the air fluid interfaceat the lower open end and can remain there trapped and alive, andwithout breaking the surface tension of the meniscus. In thosemicrowells where no force but gravity was applied to the cells duringtheir descent into the microwell, the position of the cell(s) depositedon the fluid meniscus was random (See FIG. 6A).

1.3 Analysis of Cells Deposited by Focused Alignment and Gravity

Using two or more electrodes, as shown in FIG. 2, it is possible togenerate a dielectrophoretic force acting on particles within themicrowell, and to manipulate the particles along a vertical axis of thewell during their descent from entry from the microchannel 104 at theupper open end 106 of the microwell down to the lower open end 108. Ineach pair of adjacent electrodes 116, 118, 120, a first electrode isconnected to a sinusoidal signal, while a second electrode is connectedto ground or to the same sinusoidal signal with a phase rotation of 180degrees.

An inverted microwell system was constructed and used for this study,having the features of the system shown in FIG. 2. The microwell 102 hada diameter between 100 μm and 120 μm and the vertical spacing betweeneach pair of annular electrodes 116, 118, 120 was 50 μm.

K562 cells were suspended in a 1:9 mixture of PBS and glycerol (300 mM).Electrodes 116 and 120 were connected to the same sinusoidal voltagewith a typical frequency included between 80 KHz and 100 KHz. Theamplitude of the applied voltage ranged between 3.4 V and 15 V.Electrode 118 was tied to ground. As a result of loading four cells intothe microwell while keeping the electric field activated, each of thecells was effectively forced to align along the central axis of themicrowell. On reaching the air/fluid meniscus at the lower end of themicrowell, each cell was centrally placed and in contact with the priorloaded cell(s). The aggregation is due to a combination of verticalgravity force and the horizontal component of the dielectrophoreticforce.

The open microwell was observed with an inverted microscope 140 underfluorescence conditions. When electrodes in the well were properlypolarized, one or more cell was focused to a center axis of themicrowell. The cells, so aligned during descent along the central axis,were each centrally deposited on the meniscus 122 of the microwell,forming an aggregate at the air-fluid interface and resulting in cell tocell contact.

K562 cells were labeled with a fluorescent dye and viewed with aninverted microscope. As shown in FIG. 6, while the cells were randomlydeposited on the meniscus of the microwell by gravity when no electricfield was applied (6A), the same cells deposited by combination ofvertical gravity and dielectrophoretic force formed an aggregate ofparticles deposited at a central portion of the meniscus (6B).

1.4 Analysis of Microbeads Deposited by Focused Alignment and Gravity

Polystyrene microbeads with a diameter of 10-25 μm were suspended indeionized water or glycerol at 22.5 mM and delivered by gravity tomicrowells having a diameter of about 80 μm. Electrodes 116 and 118 wereconnected to the same sinusoidal voltage with a frequency of 100 KHz.Electrode 117 was tied to ground. The focusing of aggregates of 2 beadswas analyzed and results, showing the average bead to bead distance andthe relative number of bead-bead contacts created as a function of thesignal amplitude, are reported in Table 3.

TABLE 3 Rate of bead-bead Voltage Amplitude (V) Average distance (μm)contacts 2 7.2 33% 4 2.2 90%

1.5 On-Chip Labeling of Cells Trapped on the Meniscus

The impact of changing fluid in the microchannel on the fluidsurrounding cells trapped on the meniscus at the distal end of the openmicrowell was analyzed to determine the ability of the inverted openmicrowell system to provide a single-cell centrifugation function. Acalcein staining protocol was executed on K562 cells.

K562 cells were delivered to inverted open microwells by limiteddilution and sedimentation. After cell delivery to the meniscus, themicrochannel was rinsed by flowing PBS in the microchannel for fiveminutes. A buffer containing calcein (1 mM in NaCl 0.9% w.v), a tracermolecule that becomes fluorescent when taken up into cells, wascontinuously flowed in the microchannel for 40 minutes at a constantflow rate of 9 microliters per minute. Table 4 shows the dynamic profileof fluorescence intensity monitored in single cells within themicrowells.

Cell staining was effective and demonstrated diffusion of calcein fromthe microchannel into the microwell and into the stably retained cells.The cells were surprisingly retained at the meniscus after washes, andwere viable, as demonstrated by calcein uptake. By monitoring calceinuptake on single cells, it was possible to observe different uptakeprofiles. On average, maximum uptake was achieved after 30-40 minutes.Fluorescence intensity is shown in the Table 4.

TABLE 4 Time (minutes) Cell 1 Cell 2 Cell 3 Average CV % 2 5.8 4.2 19.09.6 84 4 7.9 13.1 24.3 15.1 55 6 7.7 17.8 30.3 18.6 61 8 10.9 22.4 33.622.3 51 10 14.2 27.6 37.4 26.4 44 12 19.7 32.5 41.4 31.2 35 14 23.9 37.044.4 35.1 30 16 28.9 39.5 47.5 38.7 24 20 38.9 45.4 53.3 45.9 16 25 52.358.7 65.4 58.8 11 30 61.9 62.5 63.5 62.6 1 40 74.0 60.5 44.6 59.7 25

Example 2 2.1 Recovery of K562 Cells after Delivery and DEP Focusing

Use of the inverted open microwell system for analysis of single cellfunction permits recovery of cells determined to have desiredproperties. After trapping single cells and/or particles, for example,each in one of a plurality of microwells, for example in a microwellarray, cellular function can be analyzed by one or more bioassays. Whena particular cell is identified as having one or more desiredcharacteristic or function, the content of each microwell can berecovered and transferred to a substrate such as a microtiter plate.

A device having the features shown in FIG. 5 was developed to enablecell recovery from the inverted open microwell. To release contents ofthe microwells, a pressurized filtered gas such as nitrogen or air isinserted into the microchannel 104 in a controlled manner. For example,a pulse of pressure of about 1 bar was be applied through a connectionat the input of a normally closed electro-valve 172.

An electronic system connected to the control input of the electro-valve172 generated voltage or current pulses with a duration of about 5milliseconds. On generation of a pulse, the pressurized gas entered themicrochannel 104 via tubing connecting the microchannel input and outputport, and as a result, a fluid droplet 138 was ejected from eachmicrowell 102 and spotted onto a receptacle 126, represented by acapturing surface in some experiments or a microtiter plate in otherexperiments, where the plate was positioned under the microwell arrayand properly aligned. One or more particles and/or cells present in themicrowell were transferred to the capture surface in droplets 138 offluid.

During transfer of a cell from the microwell, it may be possible thatother cells or particles are trapped in the microchannel and may betransferred along with the cell(s) in the microwell when the pressurepulse is applied. To limit this problem, a bovine serum albumin (BSA) 1mM coating was inserted into the microchannels for 30 minutes for priorto use, in order to form a protein self-assembled monolayer. Themicrochannel was rinsed with PBS for 20 minutes before cell recovery.All of the cells had been removed from the microchannel so that only thecells contained in the microwell were transferred to the recoveryreceptacle.

2.2 Viability and Growth of Recovered Cells

One function of the inverted microwell system is to assess the functionof a single cell or cells, and to recover from the microwell or expandwithin the microwell, viable cells identified as having a desiredfunctional property. The viability of cells under activated or inactiveelectric fields and DEP as well as for different lengths of time can beassessed, for example, by monitoring the growth and expansion ofrecovered single cells or small cell clusters, for example, countingcell number daily. Cell growth can also monitored by viewing cells witha microscope.

Single K562 cells were suspended in physiological medium and cells weredelivered to microwells by sedimentation, and deposited on the air/fluidmeniscus in a plurality of microwells. After 20 minutes, the individualcells were transferred as described above by delivering a pressure pulseto the fluid in the channel, thereby causing transfer of a dropletcontaining the cell from the microwell to a well of a 96 well microtiterplate having V-shaped wells filled with RPMI growth medium supplementedwith Fetal Calf Serum. After a few hours of incubation, the single cellsedimented into the V-shaped well, and was observed with an invertedmicroscope.

The cells were incubated at 37 degrees C. with 5% CO2 and cultured forseveral days. Viewing the cell cultures each day confirmed the growth ofa clonal cell population from each of the single cells recovered. After3-5 days cell growth and expansion was demonstrated and a monoclonalcell line had been generated from the single deposited cell. Growth ofthe single cell or small aggregates of cells is shown in a series oftimed photographs and reported as a graph in FIG. 8. This studydemonstrates that deposition of cells on the meniscus of an invertedopen microwell and recovery of the cells was possible, and permittedrecovery of viable cells.

Example 3 Cell-Cell Interaction Analysis 3.1 Activity of CTL CellsAgainst Target LCL Cells

Functional live cell-cell interactions in the inverted microwell systemwere demonstrated by induction of cell lysis by T-lymphocytes on targettumor cells. Target cells, LCL cells, were marked with calcein anddelivered to microwells. T-lymphocytes (CTL) were activated against thetarget cells and delivered to the same microwells. Fluorescence of thetarget cells was monitored by fluorescence microscopy and the calceinfluorescence profile was determined as a measure of cell lysis.

Activated CTL cells induced a consistent decrease in fluorescence of thetarget cells as compared to negative control. Shown in FIG. 7, within 20minutes the observed fluorescence of the live target cell was diminishedand extinguished, demonstrating effective lysis of the target cellinduced by the CTL, an effective analysis of specific cell-to-cellinduced interaction and measured functional result, within minutes inthe inverted open microwell system.

Table 5 reports the fluorescence intensity measured on several LCLtargets delivered in inverted open microwells having a diameter of 70μm. As a control we delivered LCL cells alone. The CTL-LCL interactionwas measured in two conditions: without Human Papillomavirus (HPV)infection on target LCL and with HPV infection. In the first case nolysis is expected, while in the second case the CTL cells are expectedto recognize the target and lyse the LCL cells. Results reported in thetable show that the fluorescence intensity has a strong decrease within30 minutes for all CTL-LCL couples when the LCL was infected with HPV(cases e-h). In contrast, in only one case (d) an a-specific lysis wasobtained, representing the situation of a target cell being recognizedby the CTL cell even without HPV infection. In all the othernon-infected cases (a-c) only a physiological decrease of fluorescentsignal was observed.

TABLE 5 Time Intensity Case Cell types (minutes) (relative) a LCL(control) 30 57.9% b CTL-LCL (no HPV) 20 59.8% c CTL-LCL (no HPV) 2088.6% d CTL-LCL (no HPV) 20 23.8% e CTL-LCL 1 30 2.3% f CTL-LCL 2 206.2% g CTL-LCL 3 15 0.0% h CTL-LCL 4 15 4.5%

Example 4 Controlled Delivery of Particles in Microwells with ActiveFluid Flow

Experiments were performed to validate the functionality of the invertedopen microwell structures shown, for example in the Figures. Deviceswere produced, having microwells with a circular diameter of 100 μm, adielectric with a thickness of 50 μm and 25 μm. The thickness of eachelectrode was 9 μm. The gap 8 between electrodes 153 and 154 was 50 μm.The microchannel had a height of 150 μm and a width of 350 μm.

Polystyrene microbeads with a diameter of 10 μm were suspended inglycerol (22.5 mM). Glycerol has a density higher than water and is usedto reduce the sedimentation speed, thus limiting the adhesion ofmicrobeads to the lower surface of the microchannel. K562 cells weresuspended in a 1:9 mixture of PBS and glycerol (300 mM). This buffer hasa physiological osmolarity while the conductivity is reduced to about0.1 S/m.

In two different experiments microbeads and cells were introduced intothe microchannel and control electrodes 116, 153, 154, 155, 156, 118were polarized so as to force (trapping) or prevent (ejecting) particledelivery to the microwells. Except for electrodes tied to ground, allthe signals were sinusoids with a frequency of 100 kHz and the sameamplitude. The phase shift scheme is reported in Table 6. Thefunctionality of control electrodes was determined by a statisticalanalysis of the number of particles delivered to microwells for each ofthe particles and the cells delivered, as a function of the particlespeed and the applied voltage.

TABLE 6 Electrode Configuration 116 153 154 155 156 118 Trapping (GND)(GND) (GND) 0 π π/2 Ejecting (GND) π 0 0 π π/2

When the trapping configuration was active, the number of particlesdelivered to the microwells increased for higher signal amplitudes andlower fluid speeds. Detailed results are reported in Table 7 and Table8, where at least 50 samples were considered per each value reported.

When the ejecting configuration was active we found the structure workedproperly (i.e. no particles delivered) for peak to peak voltageamplitudes greater than 2V and for any particle speed within the range15-150 μm/s.

Flow speeds lower than 15 μm/s are typically not used as they result inparticle adhesion to the lower surface of the microchannel. The ejectingconfiguration provided higher stress on cells than the trappingconfiguration, as demonstrated by calcein release assays performed onthe cells flowing through the channel. As a consequence, when workingwith cells, peak-to-peak voltage amplitude is preferably kept below 10Vto limit stresses on the cells and maintain cell viability.

TABLE 7 Applied peak- Particle and fluid speed (μm/s) peak voltage (V)15 20 30 60 100 150 2 58% 50% 40% 0% 0% 0% 4 95% 95% 95% 57% 0% 0% 6 95%95% 85% 80% 50% 0% 8 95% 95% 100% 72% 0% 0% 10 100% 100% 96% 75% 10% 0%15 100% 100% 96% 82% 30% 0% 20 100% 100% 100% 90% 30% 0%

TABLE 8 Applied peak- Particle and fluid speed (μm/s) peak voltage (V)15 20 30 60 100 10 92% 87% 82% 75% 30% 15 97% 95% 95% 85% 35% 20 95% 92%91% 93% 50%

Example 5 Control of the Environment Outside the Inverted Open Microwellto Reduce Evaporation and Increase Cell Viability

The positive effect of controlling the environment surrounding theinverted open microwell was demonstrated by setting up a system wherethe humidity outside the microwell was brought to its saturation valueand by measuring the consistent reduction of the drag force due toevaporation.

The setup used to control the evaporation included a 384-well microtiterplate, where each microwell was filled with 100 uL of fluid, such aswater, RPMI, PBS or any buffer suitable for cell culturing stored at atemperature between 4° C. and 10° C. The inverted open microwell array114 was leaned upon the microtiter in such a way that each well of themicrotiter was aligned to an inverted open microwell, creating a closedchamber containing the fluid previously deposited in the microtiter thatcould evaporate in the closed chamber thus increasing the humidity. Thevertical distance between the pool and the meniscus 122 of the invertedopen microwell typically ranged from 0.5 mm to 5 mm. After a few minutesthe vapor pressure in the chamber reached its saturation value andprevented any further evaporation either from the microtiter and fromthe inverted open microwell.

To demonstrate the positive effect of the control of the humidity underthe microwell in reducing the evaporation in the microwell, a device 101featuring microwells with the electrode configuration shown in FIG. 2was used. Electrodes 116 and 120 were tied to ground, while electrode118 was connected to a sinusoidal signal having a frequency of 100 kHzand variable amplitude.

A suspension of K562 cells maintained at a temperature between 30° C.and 37° C. was inserted in the microchannel and the fluid flow wasstopped in order to have a single cell disposed in the microchannel overthe microwell entrance. In presence of evaporation, the cell issubjected to a drag force F_(D) directed downwards and due to the flowof fluid produced by evaporation at the air-fluid interface. Inaddition, the cell was subjected to the gravity force F_(G), thebuoyancy force F_(B) and the vertical component of the dielectrophoreticforce F_(DEPy) in such a way that:

F _(B) −F _(G) −F _(D) =F _(DEPy).

For a relatively high value of the dielectrophoretic force, the celldoes remain trapped at the entrance of the microwell and the forcesacting on the cell result to be in equilibrium. When thedielectrophoretic force is decrease, the vertical position of the celldecreases and the cell reaches a region where the electric field and thedielectrophoretic force are higher. This behavior is observed until theamplitude of the electric field reaches a critical minimum value. If theamplitude is further decreased, then the cell falls into the microwellas the dielectrophoretic force is not strong enough to counteract theother forces acting on the cell.

The vertical position of the cell of an inverted open microwellsurrounded by air was compared with or without humidity control. Thereference value for the height was the top side of the top electrode116. Positive values of the height correspond to particles remainingoutside of the microwell in the microchannel, while negative valuescorrespond to particles entering in the microwell. As reported in Table9, cell height was always higher when the humidity was controlled. Thisdemonstrates that the additional drag force F_(D) due to evaporation waseffectively removed by the presence of the setup that providedcontrolled humidity under the inverted open microwell.

TABLE 9 Cell height without Cell height with humidity control humiditycontrol Amplitude Standard Standard Theoretical (V) Mean deviation Meandeviation cell height 0.66 −28.25 0.00 −6.00 5.96 −5.9 0.75 −14.82 2.602.80 2.28 −1.2 0.85 −11.36 5.01 5.20 1.92 2.4 0.945 −7.96 7.42 7.20 1.644.9 1.035 −3.50 7.08 10.80 3.83 6.9

The control of evaporation is needed to maintain a proper physiologicalenvironment for cells trapped on the meniscus at the air-fluidinterface. In fact, the presence of evaporation would introduce anincrease in the local concentration of salts and other nutrientscontained in the medium with a consequent increase in osmolar pressure.After applying the evaporation control we measured cell viability with astandard calcein release assay where cells were stained with Calcein 1μM and obtained a signal loss of about 8% per hour, which is comparableto the well-known physiological loss, as reported in Neri et al., 2001,Clin. Diagn. Lab. Immunol., vol. 8, no. 6, pp. 1131-1135“Calcein-Acetyoxymethyl Cytotoxicity Assay: Standardization of a MethodAllowing Additional Analyses on Recovered Effector Cells andSupernatants”.

The specification includes numerous citations to published referencesand patent documents, each of which is hereby incorporated by referencein its entirety.

While the invention has been illustrated and preferred embodimentsdescribed in the forgoing specification and figures, it is understoodthat variations and changes can be made to the preferred embodimentswithout deviating from the scope and spirit of the invention, forexample, as embodied in the following claims.

1-43. (canceled)
 44. A method for producing an aggregate of specificparticles, comprising: a) inserting a first group of particles into afluid contained in a microchannel of an inverted open microwell system,the system comprising one or more microwell open at an upper end to themicrochannel and open at a lower end to a gas outside the lower end ofthe microwell; b) delivering one or more particles of the first group ofparticles from the microchannel to one or more microwells; c) depositingthe delivered one or more particles to a location on a meniscus formedat an air/fluid interface at the lower end of the one or more microwell;and d) repeating the inserting and depositing steps a), b) and c) asneeded to form a desired aggregate of specific particles positioned atthe meniscus of the microwell, where the delivered particles descendfrom the upper end of the microwell to the meniscus at the lower end ofthe microwell by gravitational force; where the inverted microwell iscapable of retaining a stable meniscus at the lower open end of themicrowell; and wherein the formed aggregate is stably retained on themeniscus during fluid flow in the microchannel.
 45. The method of claim44, where the descent of the delivered particles to the deposit locationis driven by: a) a combination of gravitational force and one or moreelectric field generated between electrodes disposed in the microwell;or b) a combination of gravitational force and one or more electricfield generated between at least one electrode disposed in the microwelland at least one electrode disposed in the upper portion of themicrowell, above the microwell, and/or in the microchannel.
 46. Themethod of claim 44, where the particles include live cells, non-livingparticles comprising biological molecules, or a combination thereof. 47.The method of claim 44, where during at least part of the descent of theone or more particle to the meniscus at the lower end of the microwell,an electric field holds the particle(s) at a central position in ahorizontal axis of the microwell as the particle(s) descend along avertical axis of the microwell, so that particles delivered to the samemicrowell are in close proximity at the meniscus.
 48. The method ofclaim 44, further comprising the step of controlling entry of one ormore particle into a microwell by: a) controlling fluid flow in themicrochannel at a sufficiently low speed to permit the one or moreparticles to enter the microwell; b) activating a pair of electrodesdisposed at the upper end of the microwell to control entry of one ormore particles into the microchannel; or c) both controlling fluid as ina) and activating electrodes as in b).
 49. The method of claim 44,further comprising one or more of the following steps: a) analyzing thedeposited particles for a detectable functional response; b) viewing oneor more of the deposited cells with a microscope; and c) recovering oneor more particle from the microwell.
 50. The method of claim 49, wherethe functional response is selected from: a) ligand/receptor binding; b)target cell lysis; c) inhibition of cell growth; d) stimulation of orinhibition of apoptosis; e) production of a biomarker f) binding to aspecific antigen; g) lysis of a target cell; h) production of anantigen-specific antibody; i) therapeutic drug effect; j) apoptosis of atarget cell; k) inhibition of cell death; l) production of a specificbiological molecule; m) cell differentiation; and n) a cell signalingevent in a target cell.
 51. The method of claim 44, wherein two or moreparticles are deposited that are cells that form a functional aggregate,the aggregate comprising the cells in direct contact or in closeproximity, and further comprising the step of analyzing the aggregate todetect a functional response.
 52. The method of claim 51, where thefunctional response is selected from: a) ligand/receptor binding; b)target cell lysis; c) inhibition of cell growth; d) stimulation of orinhibition of apoptosis; e) production of a biomarker f) binding to aspecific antigen; g) lysis of a target cell; h) production of anantigen-specific antibody; i) therapeutic drug effect; j) apoptosis of atarget cell; k) inhibition of cell death; l) production of a specificbiological molecule; m) cell differentiation; and n) a cell signalingevent in a target cell.
 53. The method of claim 44, further comprisingone or more of the following steps: a) viewing one or more of the cellsstably retained at the meniscus with a microscope positioned below thelower open end of the microwell or above the microchannel; b) recoveringone or more particle from the inverted open microwell by applying apressure pulse in the microchannel resulting in transfer of the contentsof the microwell to a substrate disposed below the lower end of the openmicrowell; and/or c) recovering the contents of the inverted openmicrowell directly onto a substrate disposed beneath the lower end ofthe open microwell.
 54. The method of claim 44, where the systemcomprises a plurality of microwells.
 55. The method of claim 54, wherethe plurality of microwells comprises an array of microwells open to thesame microchannel, and further comprising the step of transferring thecontent of multiple wells in parallel to a multi-well substrate.
 56. Themethod of claim 54, where the plurality of microwells comprises an arrayof inverted open microwells open to an atmosphere outside the devicethat is controlled for composition, humidity, temperature, pressure, andsterility.
 57. The method of claim 44, where delivering the one or moreparticles comprises: a) applying controlled potential to opposingelectrodes disposed at the top and bottom of the microchannel to createa dielectrophoretic force sufficient to cause a particle in themicrochannel to move toward a microwell; and b) applying controlledpotential to opposing electrodes disposed in the upper end of themicrowell and at the top of the microchannel to create adielectrophoretic force sufficient to cause a particle to move into themicrowell.
 58. The method of claim 57, further comprising the step ofdetecting an absence of a particle in one or more microwell using asensor and responding to the detected absence by causing electrodes atthe upper end of the microwell to generate a dielectrophoretic force tomove one or more particle into the microwell.
 59. The method of claim44, where delivering the one or more particles comprises: a) applyingcontrolled potential to opposing electrodes disposed at the top andbottom of the microchannel to create a dielectrophoretic forcesufficient to cause a particle in the microchannel to move away from amicrowell; and b) applying controlled potential to opposing electrodesdisposed in the upper end of the microwell and at the top of themicrochannel to create a dielectrophoretic force sufficient to cause aparticle to be repelled from the microwell.
 60. The method of claim 59,further comprising the step of detecting an absence of a particle in oneor more microwell using a sensor and responding to the detected absenceby causing electrodes at the upper end of the microwell to generate adielectrophoretic force to move one or more particle into the microwell.61. The method of claim 44, further comprising the steps of detectingpassage of one or more particle in the microchannel using an optical orelectrical sensor positioned along the microchannel; and responding tothe detection of the one or more particle by causing electrodes at theupper end of the microwell to generate a dielectrophoretic force to movethe particle into the microwell or to repel the particle away from themicrowell.
 62. The method of claim 61, where opposing electrodesdisposed at the top and bottom of the microchannel create adielectrophoretic force sufficient to cause the one or more particle inthe microchannel to align along an axis where the detection is performedby the sensor.
 63. The method of claim 44, comprising: a) inserting thefirst group of particles into the microchannel, where a plurality ofpaired electrodes are disposed in the microchannel; b) applying apotential to one or more of the electrode pairs to generate a series ofelectric fields sufficient to cause a particle to move in themicrochannel toward a microwell; c) trapping a particle in themicrochannel at a position above the microwell; d) applying a potentialto one or more electrode pairs in the microchannel and in the microwellto cause the particle to enter the microwell; and e) permitting theparticle to descend along a vertical axis of the microwell and todeposit at the location within the microwell.
 64. The method of claim44, further comprising the steps of: a) providing one or more reactantsto the one or more particles deposited at the location by adding the oneor more reactant to the microchannel that is in fluid communication withthe microwell; and b) correlating a change in the deposited one or moreparticles or provided one or more reactants with a specific functionalactivity possessed by the one or more particle or the reactant.
 65. Themethod of claim 44, where the fluid in the microchannel can berepeatedly changed during the method.
 66. The method of claim 44, wherethe one or more deposited particles is selected from one or more of: a)a target particle comprising a specific ligand; b) a target cellcomprising a specific ligand; c) a cytotoxic and/or cytolytic targetcell; d) a diseased cell; e) an immune cell; f) a cancer cell; g) an HIVinfected cell; h) a transgenic cell; i) an immortalized cell; and j) aB-cell.
 67. The method of claim 44, wherein the fluid is air or gas.